It’s acting as a somewhat technical term, I think. Plastic here is surely calling back to plastic art earlier in the sentence; so I would understand plastic value here as meaning value as a work of plastic art.
Plastic art, in turn, is a somewhat old-fashioned technical term for art forms involving moulded materials (clay, ceramics…), and by extension sculpture in general, as Wikipedia, the OED and various other googlable sources confirm. (This comes from the original meaning of plastic, as quoted in @mgkrebbs’ answer.)
This use of plastic value is analogous to how one might talk about the harmonic significance of a chord in a Beethoven symphony. We’re not saying that the significance harmonises nicely; we’re talking about the significance of the chord, as part of the harmony. Similarly, the author isn’t (as I understand them) talking about whether the value is plastic, can be reshaped, etc; they’re talking about the value of the work, as a piece of plastic art.